Advanced thermoelectric materials with enhanced crystal lattice structure and methods of preparation

ABSTRACT

New skutterudite phases including Ru 0 .5 Pd 0 .5 Sb 3 , RuSb 2  Te, and FeSb 2  Te, have been prepared having desirable thermoelectric properties. In addition, a novel thermoelectric device has been prepared using skutterudite phase Fe 0 .5 Ni 0 .5 Sb 3 . The skutterudite-type crystal lattice structure of these semiconductor compounds and their enhanced thermoelectric properties results in semiconductor materials which may be used in the fabrication of thermoelectric elements to substantially improve the efficiency of the resulting thermoelectric device. Semiconductor materials having the desired skutterudite-type crystal lattice structure may be prepared in accordance with the present invention by using powder metallurgy techniques. Measurements of electrical and thermal transport properties of selected semiconductor materials prepared in accordance with the present invention, demonstrated high Hall mobilities and good Seebeck coefficients. These materials have low thermal conductivity and relatively low electrical resistivity, and are good candidates for low temperature thermoelectric applications.

NOTICE

The invention described herein was made in the performance of work undera NASA contract, and is subject to the provisions of Public Law 96-517(35 U.S.C. 202) in which the Contractor has elected to retain title.

RELATED PATENT APPLICATION

This is a continuation-in-part of pending patent application Ser. No.08/189,087 filed Jan. 28, 1994 entitled "HIGH PERFORMANCE THERMOELECTRICMATERIALS AND METHODS OF PREPARATION" of same assignee, now U.S. Pat.No. 5,610,366, which is a continuation-in-part of pending patentapplication Ser. No. 08/101,901 filed Aug. 3, 1993, entitled "ADVANCEDTHERMOELECTRIC MATERIALS WITH ENHANCED CRYSTAL LATTICE STRUCTURE ANDMETHODS OF PREPARATION" of same assignee (Attorney's Docket 17083-0118).

TECHNICAL FIELD OF THE INVENTION

This invention relates to the design and preparation of semiconductormaterials having enhanced thermoelectric properties.

BACKGROUND OF THE INVENTION

The basic theory and operation of thermoelectric devices has beendeveloped for many years. Modern thermoelectric cooling devicestypically include an array of thermocouples which operate by using thePeltier effect. Thermoelectric devices are essentially small heat pumpswhich follow the laws of thermodynamics in the same manner as mechanicalheat pumps, refrigerators, or any other apparatus used to transfer heatenergy. The principal difference is that thermoelectric devices functionwith solid state electrical components (thermocouples) as compared tomore traditional mechanical/fluid heating and cooling components. Theefficiency of a thermoelectric device is generally limited to itsassociated Carnot cycle efficiency reduced by a factor which isdependent upon the thermoelectric figure of merit (ZT) of the materialsused in fabrication of the thermoelectric device.

The thermoelectric figure of merit (ZT) is a dimensionless measure ofthe effectiveness of a thermoelectric device and is related to materialproperties by the following equation:

    ZT=S.sup.2 σT/κ                                (1)

where S, σ, κ, and T are the Seebeck coefficient, electricalconductivity, thermal conductivity and absolute temperature,respectively. The Seebeck coefficient (S) is a measure of how readilythe respective carriers (electrons or holes) can change energy in atemperature gradient as they move across a thermoelectric element. Thethermoelectric figure of merit is related to the strength of interactionof charge carriers with the lattice structure and the available energystates associated with the respective materials.

The ZT may also be stated by the equation: ##EQU1##

Thermoelectric materials such as alloys of Bi₂ Te₃, PbTe and BiSb weredeveloped thirty to forty years ago. More recently, semiconductor alloyssuch as SiGe have been used in the fabrication of thermoelectricdevices. Commercially available thermoelectric materials are generallylimited to use in a temperature range between 300° K and 1300° K with amaximum ZT value of approximately one. The efficiency of suchthermoelectric devices remains relatively low at approximately five toeight percent (5-8%) energy conversion efficiency. For the temperaturerange of -100° C. to 1000° C., maximum ZT of current state of the artthermoelectric materials remains limited to values of approximately 1,except for Te-Ag-Ge-Sb alloys (TAGS) which may achieve a ZT of 1.2 to1.4 in a very narrow temperature range. Recently developed materialssuch as Si₈₀ Ge₂₀ alloys used in thermoelectric generators to powerspacecrafts for deep space missions have a thermoelectric figure ofmerit approximately equal to 0.5 from 300° C. to 1,000° C.

SUMMARY OF THE INVENTION

In accordance with the present invention disadvantages and problemsassociated with the previous design and preparation of materials used inthe manufacture of thermoelectric devices have been substantiallyreduced or eliminated. The present invention provides the ability toobtain increased efficiency from a thermoelectric device by usingmaterials with a skutterudite-type crystal lattice structure and desiredthermoelectric characteristics in fabrication of the thermoelectricdevice. Examples of semiconductor materials and compounds which aresatisfactory for use with the present invention include, but are notlimited to, IrSb₃, RhSb₃, CoSb₃, Ru₀.5 Pd₀.5 Sb₃, RuSb₂ Te, FeSb₂ Te andFe₀.5 Ni₀.5 Sb₃ and alloys of these compounds.

In accordance with one aspect of the present invention, P-typesemiconductor materials are formed from alloys of CoSb₃, RhSb₃ or IrSb₃for use in manufacturing thermoelectric devices with substantiallyenhanced operating characteristics and improved efficiency as comparedto previous thermoelectric devices.

In accordance with another aspect of the present invention, P-typesemiconductor materials are formed from skutterudite phases Ru₀.5 Pd₀.5Sb₃, RuSb₂ Te, FeSb₂ Te and Fe₀.5 Ni₀.5 Sb₃. These skutterudite phasesmay also be used for manufacturing thermoelectric devices withsubstantially enhanced operating characteristics and improved efficiencyas compared to previous thermoelectric devices.

In accordance with another aspect of the present invention, N-typesemiconductor materials are formed from alloys of CoSb₃, RhSb₃ or IrSb₃for use in manufacturing thermoelectric devices with substantiallyenhanced operating characteristics and improved efficiency as comparedto previous thermoelectric devices. In accordance with another aspect ofthe present invention, N-type semiconductor materials are formed fromskutterudite phases Ru₀.5 Pd₀.5 Sb₃, RuSb₂ Te, FeSb₂ Te and Fe₀.5 Ni₀.5Sb₃. These skutterudite phases may also be used for manufacturingthermoelectric devices with substantially enhanced operatingcharacteristics and improved efficiency as compared to previousthermoelectric devices.

An important technical advantage of the present invention includes theuse of gradient freezing techniques in the preparation of semiconductormaterials such as IrSb₃, RhSb₃ and CoSb₃. The use of gradient freezetechniques in accordance with the present invention produces a large,single crystal of semiconductor material having a skutterudite latticestructure. A Bridgman Two-Zone furnace and a sealed container have beenmodified for use in preparation of semiconductor materials in accordancewith the present invention.

Another important technical advantage of the present invention includesthe use of liquid-solid phase sintering techniques in the preparation ofsemiconductor materials such as IrSb₃, RhSb₃ and CoSb₃. The use ofliquid-solid phase sintering techniques in accordance with the presentinvention produces a large ingot of semiconductor material having askutterudite lattice structure. An isothermal furnace and a sealedcontainer have been modified for use in preparation of semiconductormaterials in accordance with the present invention.

Another important technical advantage of the present invention includesthe use of powder metallurgy techniques in the preparation ofskutterudite phases Ru₀.5 Pd₀.5 Sb₃, RuSb₂ Te, FeSb₂ Te and Fe₀.5 Ni₀.5Sb₃. The use of powder metallurgy techniques produces a polycrystallinesemiconductor material having a skutterudite lattice structure.

Another aspect of the present invention includes manufacturing athermoelectric device with P-type thermoelectric elements formed frommaterials such as Ru₀.5 Pd₀.5 Sb₃, RuSb₂ Te, FeSb₂ Te, Fe₀.5 Ni₀.5 Sb₃,CoSb₃, RhSb₃ or IrSb₃ and N-type thermoelectric elements formed fromSi₈₀ Ge₂₀ or alloys of bismuth (Bi), arsenic (As), antimony (Sb),selenium (Se), and tellurium (Te), the salts of lead with chalcogenssulphur (S), tellurium (Te) and selenium (Se).

The present invention allows the manufacture of thermoelectric energyconversion devices such as electrical power generators, coolers, andthermocouples or temperature detectors with high ZT and associatedincreased efficiency. By the use of semiconductor phases such as Ru₀.5Pd₀.5 Sb₃, RuSb₂ Te, FeSb₂ Te, and Fe₀.5 Ni₀.5 Sb₃, semiconductorcompounds IrSb₃, RhSb₃ and CoSb₃, and alloys of these compounds whichhave been prepared in accordance with the present invention, the overallefficiency of a thermoelectric device may be substantially enhanced.

A further important technical advantage includes the use ofsemiconductor materials prepared in accordance with the presentinvention in the manufacture of a radioisotope thermoelectric generator(RTG) to substantially enhance the associated system efficiency. Suchthermoelectric devices may be used in space power systems. Otherthermoelectric devices manufactured from semiconductor materialsfabricated in accordance with the present invention may be used in wasteheat recovery systems, automobiles, remote power generators and sensorsand coolers for advanced electronic components such as field effecttransistors.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and itsadvantages, reference is now made to the following description taken inconjunction with the accompanying drawings in which:

FIG. 1 is an isometric drawing of a thermoelectric device which may bemanufactured from materials incorporating the present invention;

FIG. 2 is an electrical schematic drawing of the thermoelectric deviceof FIG. 1;

FIG. 3 is an isometric representation of a skutterudite-type crystallattice structure associated with semiconductor materials which havebeen fabricated in accordance with the present invention;

FIG. 4 is an iridium antimony phase diagram;

FIG. 5a is a schematic drawing in elevation and in section with portionsbroken away showing a Bridgman Two-Zone furnace which may be used toprepare semiconductor materials using gradient freeze techniques inaccordance with the present invention;

FIG. 5b is a graph showing the temperature gradient associated withgrowing large, single crystals of semiconductor materials having askutterudite lattice structure in accordance with the present invention;

FIG. 6 is a schematic drawing in elevation and in section with portionsbroken away showing an isothermal furnace which may be used in preparingingots by liquid-solid phase sintering of semiconductor materials havinga skutterudite lattice structure in accordance with the presentinvention;

FIG. 7 is a graph showing typical electrical resistivity as an inversefunction of temperature associated with semiconductor materials preparedin accordance with the present invention;

FIG. 8 is a graph showing typical Hall mobility values as an inversefunction of temperature associated with semiconductor materials preparedin accordance with the present invention;

FIG. 9 is a graph showing typical Seebeck coefficients as a function oftemperature for semiconductor materials prepared in accordance with thepresent invention;

FIG. 10 is a graph showing thermal conductivity as a function oftemperature for semiconductor materials prepared in accordance with thepresent invention as compared with presently available thermoelectricmaterials;

FIG. 11 is a graph showing the performance of a multiple stagethermoelectric cooler fabricated in part from semiconductor materialsincorporating the present invention as compared to a multiple stagethermoelectric cooler fabricated from presently available semiconductormaterials;

FIG. 12 is a graph showing typical electrical resistivity as an inversefunction of temperature associated with semiconductor materials preparedin accordance with the present invention;

FIG. 13 is a graph showing typical Hall mobility values as an inversefunction of temperature associated with semiconductor materials preparedin accordance with the present invention;

FIG. 14 is a graph showing typical Hall carrier concentrations as afunction of temperature for semiconductor materials prepared inaccordance with the present invention;

FIG. 15 is a graph showing typical Seebeck coefficients as a function oftemperature for semiconductor materials prepared in accordance with thepresent invention;

FIG. 16 is a graph showing thermal conductivity as a function oftemperature for semiconductor materials prepared in accordance with thepresent invention as compared with presently available thermoelectricmaterials;

FIG. 17 illustrates possible substitutions on the cation site to formternary compounds from binary skutterudites;

FIG. 18 illustrates possible substitutes on the anion site to formternary compounds from binary skutterudites; and

FIG. 19 illustrates substitutions that can be made on both the anion andcation sites to form ternary compounds from binary skutterudites.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiments of the present invention and its advantagesare best understood by reference to FIGS. 1 through 19 of the drawings,like numerals being used for like corresponding parts of the variousdrawings.

Thermoelectric device 20, as shown in FIGS. 1 and 2, may be manufacturedfrom semiconductor materials and compounds which have been prepared inaccordance with the present invention. The use of such semiconductormaterials will substantially increase energy conversion efficiency.Thermoelectric device 20, as shown, may be used as a heater and/or acooler. Thermoelectric device 20 is preferably manufactured with aplurality of thermoelectric elements (sometimes referred to as"thermocouples") 22 disposed between cold plate 24 and hot plate 26.Electrical power connections 28 and 29 are provided to allow attachingthermoelectric device 20 to an appropriate source of DC electricalpower. If thermoelectric device 20 were redesigned to function as anelectrical power generator, electrical connections 28 and 29 wouldrepresent the output terminals from such a power generator operatingbetween hot and cold temperature sources (not shown).

FIG. 2 is a schematic representation of electrical circuit 30 associatedwith thermoelectric device 20. Electrical circuit 30 is also typical ofelectrical circuits associated with using thermoelectric elements orthermocouples to convert heat energy into electrical energy. Suchelectrical power generators may be used in waste heat recovery systems(not shown), space power systems (not shown) and radioisotope powergenerators (not shown).

Electrical circuit 30, which is typical for a single stagethermoelectric device such as thermoelectric device 20, generallyincludes two dissimilar materials such as N-type thermoelectric elements22a and P-type thermoelectric elements 22b. Thermoelectric elements 22are typically arranged in an alternating N-type element to P-typeelement configuration. In many thermoelectric devices, semiconductormaterials with dissimilar characteristics are connected electrically inseries and thermally in parallel.

N-type semiconductor materials have more electrons than necessary tocomplete a perfect molecular lattice structure. P-type semiconductormaterials have fewer electrons than necessary to complete a latticestructure. The "missing electrons" are sometimes referred to as "holes."The extra electrons in the N-type semiconductor material and the holesin the P-type semiconductor material (hereinafter referred to as"carriers") are the agents, coupled with lattice vibrations (referred toas "phonons"), which transport or move heat energy between cold side orcold plate 24 and hot side or hot plate 26 of thermoelectric elements22. Ceramic materials are frequently used in the manufacture of plates24 and 26 which define in part the cold side and hot side, respectively,of thermoelectric device 22.

In thermoelectric device 20, alternating thermoelectric elements 22 ofN-type and P-type semiconductor materials have their ends connected in aserpentine fashion by electrical conductors such as 32, 34 and 36.Conductors 32, 34 and 36 are typically metallizations formed on theinterior surfaces of plates 24 and 26. Commercially availablethermoelectric coolers may include two metallized ceramic plates withP-type and N-type elements of bismuth telluride alloys soldered betweenthe ceramic plates.

When DC electrical power from power supply 38 is applied tothermoelectric device 20 having an array of thermoelectric elements 22,heat energy is absorbed on cold side 24 of thermoelectric elements 22.The heat energy passes through thermoelectric elements 22 and isdissipated on hot side 26. A heat sink (sometimes referred to as the"hot sink", not shown) may be attached to hot plate 26 of thermoelectricdevice 20 to aid in dissipating heat from thermoelectric elements 22 tothe adjacent environment. In a similar manner a heat sink (sometimesreferred to as a "cold sink", not shown) may be attached to cold side 24of thermoelectric device 20 to aid in removing heat from the adjacentenvironment. Thermoelectric device 20 may sometimes be referred to as athermoelectric cooler. However, since thermoelectric devices are a typeof heat pump, thermoelectric device 20 may function as either a cooler,heater, or power generator.

Semiconductor materials (sometimes referred to as "semiconductorcompounds") CoSb₃, RhSb₃ and IrSb₃ have been prepared in accordance withthe present invention in the form of a large, single or poly crystalwith a skutterudite lattice structure 40 as shown in FIG. 3.Additionally, polycrystalline phases having the skutterudite crystallattice 40 shown in FIG. 3 such as Ru₀.5 Pd₀.5 Sb₃, RuSb₂ Te, FeSb₂ Teand Fe₀.5 Ni₀.5 Sb₃ have also been prepared. As will be explained laterin more detail, such semiconductor materials are preferably prepared inan antimony rich environment.

The skutterudite crystal lattice structure is defined in part by a unitcell with eight members having the formula AB₃. More generally, theskutterudite crystal lattice structure is defined by thirty-two atomiccrystallographic sites where eight positions may be occupied by metalsand twenty-four positions may be occupied by nonmetals. The unit celldimension for skutterudites ranges from 7.7 to 9.4. Angstroms.Skutterudite phase Ru₀.5 Pd₀.5 Sb₃ has a lattice constant that has beenexperimentally measured at 9.2944 Angstroms while phase Fe₀.5 Ni₀.5 Sb₃has a lattice constant that has been experimentally measured at 9.1001Angstroms. Alloys having the formula Co_(1-x-y) Rh_(x) Ir_(y) Sb₃, where0≦x≦1 and 0≦y≦1 have also been prepared using the present invention.

Skutterudite-type crystal lattice structure 40 includes a cubic latticeof metal atoms 42. Metal atoms 42 are preferably selected from the groupconsisting of ruthenium, palladium, iron, nickel, cobalt, iridium,and/or rhodium. A four-member planary ring 44 of non-metal atoms 46 isdisposed within the cubic lattice structure. Planary rings 44 preferablyinclude four atoms of antimony. Each metal atom 42 has six neighboratoms 42. Non-metal atoms 46 have two adjacent non-metal atoms 46 andtwo metal atoms 42. The covalent bonding associated withskutterudite-type crystal lattice structure 40 provides high holemobility and low thermal conductivity.

Semiconductor materials having skutterudite-type crystal latticestructure 40 may be doped with selected impurities to produce N-typethermoelectric elements 22a and P-type thermoelectric elements 22b. Forexample, adding platinum (Pt) as a doping agent with IrSb₃ semiconductormaterial produced N-type thermoelectric elements. If desired,thermoelectric device 20 may be manufactured with N-type thermoelectricelements 22b fabricated from IrSb₃ and N-type thermoelectric elements22a fabricated from previously available semiconductor materials such asalloys of Bi, As, Sb, Te, salts of lead with chalcogen elements,sulphur, tellurium and selenium.

Skutterudite phases Ru₀.5 Pd₀.5 Sb₃, RuSb₂ Te, FeSb₂ Te and Fe₀.5 Ni₀.5Sb₃ may also be doped with selected elements to produce N-typethermoelectric elements 22a and P-type thermoelectric elements 22b.These phases may be doped with selected elements such as silicon orgermanium to produce P-type thermoelectric elements 22b. In addition,the skutterudite phase Ru₀.5 Pd₀.5 Sb₃ may be made into an N-typethermoelectric element 22a by substituting some amount of Fe for Ruand/or some amount of Ni or Pt for Pd. The carrier concentration ofRu₀.5 Pd₀.5 Sb₃ can also be adjusted by using excess Ru and a deficit ofPd. Similarly, the skutterudite phase Fe₀.5 Ni₀.5 Sb₃ may be made intoan N-type thermoelectric element 22a by substituting some amount of Rufor Fe and/or some amount of Pd or Pt for Ni. The carrier concentrationof Fe₀.5 Ni₀.5 Sb₃ can also be adjusted by using excess Fe and a deficitof Ni. Similar substitutions can be made in other skutterudite phases.If desired, thermoelectric device 20 may be manufactured with P-typethermoelectric elements 22b fabricated from Ru₀.5 Pd₀.5 Sb₃, RuSb₂ Te,FeSb₂ Te, or Fe₀.5 Ni₀.5 Sb₃ and N-type thermoelectric elements 22afabricated from previously available semiconductor materials such asalloys of Bi, As, Sb, Te, salts of lead with chalcogen elements, sulfur,tellurium, and selenium.

Other ternary compounds can be formed such as CoGe₁.5 Se₁.5 and CoGe₁.5S₁.5. Such ternary compounds can be formed by simultaneous substitutionof the transition metal (T) atom or pnicogen (Pn) atom in a binaryskutterudite (TPn₃) by elements on the left and on the right of theseatoms in the periodic table of the elements. This substitution resultsin an isoelectronic phase.

FIGS. 17-19 summarize several possible substitutions to form ternarycompounds from binary skutterudites. For example, as illustrated in FIG.17, for substitutions on the cation site, a ternary compound can beformed of the formula T'₀.5 T"₀.5 Pn₃ where T'₀.5 may be selected fromthe group consisting of Fe, Ru, and Os. T"₀.5 may be selected from thegroup consisting of Ni, Pd, and Pt. Pn may be selected from the groupconsisting of P, As and Sb. Known skutterudite phases formed by suchsubstitutions include Fe₀.5 Ni₀.5 As₃. The thermoelectric properties ofthis phase were previously unknown. Newly discovered skutterudite phasesformed by such substitutions and prepared using powder metallurytechniques include Fe₀.5 Ni₀.5 Sb₃, Fe₀.5 Pd₀.5 Sb₃, Fe₀.5 Pt₀.5 Sb₃,Ru₀.5 Ni₀.5 Sb₃, Ru₀.5 Pd₀.5 Sb₃, and Ru₀.5 Pt₀.5 Sb₃.

Alternatively, as illustrated in FIG. 18, substitutions may also be madeon the anion site to form a ternary compound having the formula TA₁.5B₁.5. T may be selected from the group consisting of Co, Rh and Ir; Amay be selected from the group consisting of Si, Ge, Sn and Pb. B may beselected from the group consisting of S, Se, and Te. Known skutteruditephases formed by such substitutions include CoGe₁.5 S₁.5, CoGe₁.5 Se₁.5,RhGe₁.5 S₁.5, IrGe₁.5 S₁.5, IrGe₁.5 Se₁.5, and IrSn₁.5 S₁.5. Thethermoelectric properties of these phases were previously unknown. Newlydiscovered skutterudite phases formed by such substitutions and preparedusing powder metallurgy techniques include CoSn₁.5 Te₁.5 and IrSn₁.5Te₁.5.

Finally, other substitutions may be made as illustrated in FIG. 19. Thecompounds RuSb₂ Te and PtSnSb₂ contain substitutions on both a cationsite and an anion site. Thus, skutterudite phases can be formed bysubstituting atoms at the cation site, anion site, or both, so long asthe valence electron count remains constant. Known skutterudite phasesformed by such substitutions include PtSn₁.2 Sb₁.8. The thermoelectricproperties of this phase were previously unknown. Newly discoveredskutterudite phases formed by such substitutions and prepared usingpowder metallurgy techniques include FeSb₂ Se, FeSb₂ Te, RuSb₂ Se, andRuSb₂ Te. These phases as well as those phases discussed in connectionwith FIGS. 17 and 18 may all be used as semiconductors forthermoelectric devices.

Large, single crystals of semiconductor materials have been prepared inaccordance with the present invention using both gradient freezetechniques and liquid-solid phase sintering techniques. The phasediagram for iridium-antimony, as shown in FIG. 4, demonstrates thatgrowth of the desired skutterudite-type crystal lattice structure isvery difficult. Such crystal growth is preferably initiated from anarrow range of compositions. In addition, the liquid crystal curve inthe region forming IrSb₃ is relatively sharp which further increases thedifficulty of separating liquid and solid phases during crystallization.

Depending upon the type of semiconductor material and the relationshipof cobalt, rhodium and iridium, either gradient freeze techniques orliquid-solid phase sintering techniques may be used to produce thedesired skutterudite-type crystal lattice structure. For somesemiconductor materials such as CoSb₃ and RhSb₃, gradient freezetechniques have produced the desired large, single crystal. For othersemiconductor materials such as single phase IrSb₃, liquid-solid phasesintering techniques have produced the desired large, single crystals.

Large, single crystals of semiconductor materials with the desiredskutterudite lattice structure 40 may be grown by using gradient freezetechniques and furnace 50 as shown in FIG. 5a. Furnace 50 is frequentlyreferred to as a Bridgman Two-Zone furnace. Furnace 50 includes housing52 with a first or upper heater assembly 54 and a second or lower heaterassembly 56. Housing 52 defines in part chamber 60. Thermal baffle 58 ispreferably disposed between first heater assembly 54 and second heaterassembly 56 intermediate chamber 60. Various components which comprisefurnace 50 are preferably disposed vertically within chamber 60 ofhousing 52.

As shown in FIG. 5a, housing 52 includes end closure 62 which seals theupper portion of chamber 60 and end closure 64 which seals the lowerportion of chamber 60. Quartz rod 66 may be vertically disposed withinchamber 60. Container 68 is preferably secured to one end of rod 66adjacent to thermal baffle 58.

The lower portion 70 of container 68 is preferably pointed or taperedwith respect to rod 66. Various types of containers 68 may besatisfactorily used with the present invention. A sealed quartz crystalor quartz ampoule has been found satisfactory for use with furnace 50.If desired, housing 52 and end closure 64 may be modified to allow aconveyor (not shown) with a plurality of rods 66 and containers 68 to bepassed sequentially through furnace 50.

Elements such as Co, Rh and Sb powders which will be formed into thedesired semiconductor material using furnace 50 are preferably sealedwithin container 68 under a vacuum. Pointed or tapered end 70 ofcontainer 68 is attached to quartz rod 66 and disposed vertically withinchamber 60. Tapered end 70 and its attachment to rod 66 cooperate tomaintain the desired temperature gradients in container 68. Furnace 50is then heated to establish the temperature gradients 68 and 67 as shownin FIG. 5b. Various temperature gradients may be used depending upon theelements placed within container 68 to produce the desired semiconductormaterial.

Samples of CoSb₃ and RhSb₃ were directionally crystallized fromnon-stoichiometric melts 72 rich in antimony. Crystals of CoSb₃ andRhSb₃ were grown with an axial temperature gradient of about 50° C./cmand a growth rate about 1 mm/day. The crystals of CoSb₃ and RhSb₃ wereapproximately 10 mm long and 6 mm in diameter. The average density ofthe CoSb₃ crystals was measured and found to be 99.7% of the theoreticaldensity (7.69 g/cm³). The average density of the RhSb₃ crystals wasmeasured and found to be 99.5% of the theoretical density (7.69 g/cm³).Crystals of the Ir_(x) Co_(1-x) Sb₃ solid solutions were alsosuccessfully grown by the gradient freeze technique from antimony-richmelts in furnace 50.

Large ingots of semiconductor materials with a skutterudite latticestructure may also be prepared by using liquid-solid phase sinteringtechniques and furnace 80 as shown in FIG. 6. Furnace 80 may be referredto as an isothermal furnace as compared to furnace 50 which has twodifferent temperature zones. Furnace 80 includes housing 82 with heaterassembly 84 disposed therein. Housing 82 defines in part chamber 90.Various components which comprise furnace 80 are preferably verticallydisposed within chamber 90 of housing 82.

As shown in FIG. 6, housing 82 includes end closure 92 which seals theupper portion of chamber 90 and end closure 94 which seals the lowerportion of chamber 90. Quartz rod 66 is preferably disposed verticallywithin chamber 90. Container 68 is preferably secured within chamber 90intermediate end closures 94 and 92 at approximately the mid point ofchamber 90.

The elements such as Ir, Rh, Co and Sb which will be used to form thedesired semiconductor material may be sealed within container 68. Thelower portion 70 of container 68 is preferably pointed or tapered withrespect to quartz rod 66. The relationship of tapered end 70 with quartzrod 66 cooperate to maintain the desired temperature gradient incontainer 68 during growth of the skutterudite-type crystal structure40. Various types of containers 68 may be satisfactorily used with thepresent invention. A sealed quartz ampoule has been found satisfactoryfor use with the present invention. As previously noted for furnace 50,housing 82 and end closure 94 may be modified to allow a conveyor (notshown) to pass a plurality of rods 66 and containers 68 sequentiallythrough furnace 80.

Liquid-solid phase sintering techniques have been used to prepare large,single crystals of semiconductor material IrSb₃ and also some alloys of(Ir_(1-x-y) Rh_(x) Co_(y)) Sb₃ solid solutions using furnace 80 andcontainer 68. The samples produced had good semiconducting propertiesincluding exceptional P-type Hall mobilities as high as 7725 cm².V⁻¹.s⁻¹at room temperature. The crystals were approximately 10 mm long and 6 mmin diameter.

The liquid-solid phase sintering technique used to produce IrSb₃ and(Ir_(1-x-y) Rh_(x) Co_(y))Sb₃ solid solutions included placing a firstlayer 98 of elemental iridium, cobalt and/or rhodium powders togetherwith a second layer 100 of antimony shots in a container sealed undervacuum. In the case of the preparation of a solid solution (Ir_(1-x-y)Rh_(x) Co_(y))Sb₃, the powders of iridium, cobalt and rhodium in thedesired amount were loaded in plastic vials, mixed and shaken in a mixermill for about thirty minutes. The container 68 with material layers 98and 100 was then held vertical and heated in furnace 80 as illustratedin FIG. 6. Several different reaction times and temperatures were tried.For some mixtures, the best results were obtained with a reaction timeof 24 hours at a temperature of 1000° C. Under these conditions, theresulting crystals were the most dense.

Transport properties measurements performed on samples of CoSb₃, RhSb₃and (Ir_(1-x-y) Rh_(x) Co_(y))Sb₃ prepared using the previouslydescribed procedures and apparatus demonstrated excellent semiconductingand thermoelectric properties. For example, the compounds withskutterudite crystallographic structure 40 had exceptional high P-typeHall mobilities. Room temperature values as high as 7725 cm².v⁻¹.s⁻¹were measured on a RhSb₃ sample at a doping level of 2.4 10¹⁸ cm⁻³.Although linked to the particular crystal structure of this compound,this high value is also a result of the good quality of the sample. Forexample, lower mobilities were measured on hot-pressed RhSb₃ samplessuch as a maximum value of 1500 cm².V⁻¹.s⁻¹. Mobility as high as 1732cm².V⁻¹.s⁻¹ were measured on P-type CoSb₃ single crystals compared to amaximum value of 290 cm².V⁻¹.s⁻¹ for samples prepared by otherprocedures. IrSb₃ samples also possessed high P-type Hall mobilities ashigh as 1241 cm².V⁻¹.s⁻¹ at a doping level of 7.2 10¹⁸ cm⁻³.

Semiconductor compounds and related solid solutions have also beenprepared by sintering elemental powders of iridium, cobalt, rhodium andantimony in various ratios. Completion of the reaction was achieved intimes as short at six hours at a temperature of 600° C. These powdershave been successfully hot-pressed under specific conditions into largeingots (not shown).

Powders of IrSb₃ compound and several compositions of the (Ir_(1-x-y)Rh_(x) Co_(y))Sb₃ solid solutions were also prepared in relatively shorttimes (as low as six hours) by hot press sintering elemental powders oriridium, cobalt, antimony and rhodium. Several different mixtures ofelemental powders were successfully hot-pressed in a graphite die (notshown) into dense ingots about 15 mm long and 6 mm in diameter. Theresulting compounds demonstrated desirable thermoelectric properties.

Doping of the elemental powders can be achieved by introducing thedesired amount of dopant in the initial powder load. By usingcommercially available hot presses and graphite die containers, thisprocess is quick, cost effective and may be easily adapted to industrialmanufacturing of large quantities of (Ir_(1-x-y) Rh_(x) Co_(y))Sb₃samples of different compositions and doping level.

Ternary skutterudite-type phases can be obtained by simultaneoussubstitution of the transition-metal or pnicogen atom in a binaryskutterudite by elements on the left and on the right of this atom inthe periodic table. The result is an isoelectronic phase. For example,the ternary phases CoGe₁.5 Se₁.5 and CoGe₁.5 S₁.5 can be derived fromthe binary compound CoAs₃. The existence of a compound Fe₀.5 Ni₀.5 Sb₃formed by the substitution of the Co atom by Fe and Ni in the compoundsCoSb₃ has also been reported but thermoelectric properties of this phasewere previously unknown.

Skutterudite phases Ru₀.5 Pd₀.5 Sb₃ and Fe₀.5 Ni₀.5 Sb₃ may be preparedwith powder metallurgy techniques using a single zone isothermalfurnace. For example, single phase, polycrystalline samples of Ru₀.5Pd₀.5 Sb₃ can be prepared by direct combination of the elements.Ruthenium (99.997%), palladium (99.9%) and antimony (99.9999%) powdersare mixed in stoichiometric ratio in a plastic vial before being loadedand sealed in a quartz ampoule under vacuum. The ampoule is then heatedfor eight days at 600° C. Next, the product is removed from the ampouleand crushed in an agate mortar. The mixture is then reloaded in a secondquartz ampoule, and heated for four days at 550° C. The skutteruditephase Feo₀.5 Ni₀.5 Sb₃ may be prepared similarly. Iron (99.999%), nickel(99.996%) and antimony (99.9999%) powders are mixed in stoichiometricratio in a plastic vial and then loaded in a quartz ampoule undervacuum. The ampoule is then heated for four days at 750° C., the mixtureis crushed and then made subject to a second annealing for four days at550° C.

After synthesis of these powders, high density samples can be preparedby hot pressing. Powders of the skutterudite phases Ru₀.5 Pd₀.5 Sb₃ andFe₀.5 Ni₀.5 Sb₃ can be hot pressed into cylinders approximately 6 mm indiameter and 6 mm long. Hot pressing may be conducted, for example, at apressure of about 20,000 psi and a temperature of 500° C. for two hours.

Other skutterudite phases may also be prepared similarly using powdermetallurgy techniques. Examples of newly discovered phases that may beprepared in this manner include RuSb₂ Te, FeSb₂ Te, RuSb₂ Se, FeSb₂ Se,Ru₀.5 Pt₀.5 Sb₃, Ru₀.5 Ni₀.5 Sb₃, Fe₀.5 Pt₀.5 Sb₃, Fe₀.5 Pd₀.5 Sb₃,IrSn₁.5 Te₁.5 and CoSn₁.5 Te₁.5.

Semiconductor compounds of CoSb₃, RhSb₃ and IrSb₃ with skutterudite-typecrystal lattice structure prepared in accordance with the presentinvention have demonstrated the characteristics shown in the followingTable I.

                  TABLE I                                                         ______________________________________                                        Material      CoSb.sub.3                                                                              RhSb.sub.3                                                                              IrSb.sub.3                                  Melting Point (°C.)                                                                  850       900       1140                                        Type of formation                                                                           peritectic                                                                              peritectic                                                                              peritectic                                  from the melt                                                                 Structure type                                                                              cubic IM3 cubic IM3 cubic IM3                                   Prototype     CoAs.sub.3                                                                              CoAs.sub.3                                                                              CoAs.sub.3                                  Number of atoms/unit                                                                        32        32        32                                          cell                                                                          Lattice parameter (Å)                                                                    9.0347    9.2322    9.2533                                     Density (g · cm.sup.-3)                                                            7.69      7.96      9.32                                        Thermal expansion                                                                           6.36 10.sup.-6                                                                          7.28 10.sup.-6                                                                          6.87 10.sup.-6                              coefficient (C.sup.-1)                                                        Energy bandgap (eV)                                                                         0.5       0.8       1.17                                        Conductivity type                                                                           p         p         p                                           Electrical    0.55      0.34      0.70                                        resistivity (mΩ · cm)                                          at 25° C.                                                              Hall mobility 1732      7725      1241                                        (cm.sup.-2 · V.sup.-1 · s.sup.-1) at 25° C.          Hall carrier con-                                                                           6.5 × 10.sup.18                                                                   2.4 × 10.sup.18                                                                   7.2 × 10.sup.18                       centration (cm.sup.-3) at                                                     25° C.                                                                 Seebeck coefficient                                                                         150       90        110                                         (μV · κ.sup.-1) at 25° C.                            ______________________________________                                    

Semiconductor compounds of Ru₀.5 Pd₀.5 Sb₃ and Fe₀.5 Ni₀.5 Sb₃ withskutterudite-type crystal lattice structures prepared in accordance withthe present invention have demonstrated the characteristics shown in thefollowing Table II.

                  TABLE II                                                        ______________________________________                                        Material          Fe.sub.0.5 Ni.sub.0.5 Sb.sub.3                                                          Ru.sub.0.5 Pd.sub.0.5 Sb.sub.3                    Melting Point (°C.)                                                                      729       647                                               Preparation method                                                                              powder    powder                                                              metallurgy                                                                              metallurgy                                        Structure type    cubic (Im3)                                                                             cubic (Im3)                                       Prototype         CoAs.sub.3                                                                              CoAs.sub.3                                        Number of atoms/unit cell                                                                       32        32                                                Lattice parameter (Å)                                                                         9.0904    9.2960                                          Mass density (g · cm.sup.-3)                                                             7.462     7.746                                           Energy bandgap (eV)                                                                             ˜0.2                                                                              ˜0.6                                        Conductivity type p         p                                                 Electrical resistivity at                                                                        1.69      1.58                                             25° C. (mΩm)                                                     Hall mobility at 25° C.                                                                  27.1      37.4                                              (cm.sup.-2 · V.sup.-1 s.sup.-1)                                      Carrier concentration at                                                                        1.36 × 10.sup.20                                                                  1.06 × 10.sup.20                            25° C. (cm.sup.-3)                                                     Seebeck coefficient at 25° C.                                                            13.7      20.1                                              (μV · κ.sup.-1)                                             Thermal conductivity at                                                                         29.6       7.2                                              25° C. (mW · cm.sup.-1 κ.sup.-1)                        ______________________________________                                    

Semiconductor compounds of RuSb₂ Te and FeSb₂ Te with skutterudite typecrystal lattice structures prepared in accordance with the presentinvention have demonstrated the characteristics shown in the followingTable III.

                  TABLE III                                                       ______________________________________                                        Material           RuSb.sub.2 Te                                                                            FeSb.sub.2 Te                                   Melting Point (°C.)                                                                       810        556                                             Preparation method powder     powder                                                             metallurgy metallurgy                                      Structure type     cubic      cubic                                                              (Im3)      (Im3)                                           Prototype          CoAs.sub.3 CoAs.sub.3                                      Numbers of atoms/unit cell                                                                       32         32                                              Lattice parameter (Å)                                                                          9.2681     9.1120                                        Mass density (g · cm.sup.-3)                                                              7.869      7.746                                         Energy bandgap (eV)                                                                              >0.7       "0.6.sup.                                       Conductivity type  p          p                                               Electrical resistivity at                                                                         1.61       1.59                                           25° C. (mΩcm)                                                    Hall mobility at 25° C. (cm.sub.2 V.sup.-1 s.sup.-1)                                      43.1        7.7                                            Carrier concentration at                                                                         9.02 × 10.sup.19                                                                   4.95 × 10.sup.20                          25° C. (cm.sup.-3)                                                     Seebeck coefficient at                                                                           16.7       54.7                                            25° C. (μV · κ.sup.-1)                               Thermal conductivity at                                                                          33.1       23.2                                            25° C. (mW · cm.sup.-1 κ.sup.-1)                        ______________________________________                                    

FIG. 7 is a graphical representation of typical electrical resistivityvalues as a function of inverse temperature for semiconductor compoundsCoSb₃, RhSb₃ and IrSb₃ having skutterudite-type crystal latticestructure 40. FIG. 8 is a graphical representation of typical Hallmobility values as a function of inverse temperatures for semiconductorcompounds CoSb₃, RhSb₃ and IrSb₃ having skutterudite-type crystallattice structure 40. FIG. 9 is a graphic representation of typicalSeebeck coefficient values as a function of temperature forsemiconductor compounds CoSb₃ and IrSb₃ having skutterudite-type crystallattice structure 40.

FIG. 10 comparisons of thermal conductivity as a function of temperaturefor semiconductor materials IrSb₃ and Ir₀.75 Co₀.25 Sb₃ prepared inaccordance with the present invention as compared with previouslyavailable thermoelectric materials SiGe alloys and PbTe alloys. Curves110 and 112 show thermal conductivity measured for semiconductormaterials SiGe and PbTe respectively. Curves 114 and 116 are based onthermal conductivity measurements for semiconductor materials IrSb₃ andIr₀.75 Co₀.25 Sb₃ respectively.

A multiple stage thermoelectric cooler (not shown) is typicallyfabricated by vertically stacking two or more single stagethermoelectric devices 20. Each ascending thermoelectric device willhave fewer thermoelectric elements or thermocouples 22. A multiple stagethermoelectric cooler is therefore typically pyramid shaped because thelower stage requires more thermoelectric elements to transfer the heatdissipated from the upper stage in addition to the heat pumped from theobject being cooled by the multiple stage thermoelectric cooler. Fieldeffect transistors are often cooled from 300° C. to 125° C. by usingsuch multiple stage thermoelectric coolers.

P-type semiconductor material IrSb₃ prepared in accordance with thepresent invention may be used to provide a portion of thermoelectricelements 22. Currently available N-type semiconductor materials Bi₂ Te₃may be used to provide another portion of thermoelectric elements 22.The resulting combination substantially enhances the performance ofthermoelectric device 20. This combination of P-type and N-typesemiconductor materials is particularly useful in the 100° C. to 400° C.temperature range. FIG. 11 is a graphical representation showing theincrease in multiple stage thermoelectric cooler performance resultingfrom the use of P-type semiconductor materials IrSb₃. FIG. 11 shows thecoefficient of performance (COP_(MAX)) as a function of the number ofstages in each thermoelectric cooler.

FIG. 12 is a graphical representation of typical electrical resistivityvalues as a function of inverse temperature for two samples ofskutterudite phase Ru₀.5 Pd₀.5 Sb₃ prepared in accordance with theinvention and one sample of Fe₀.5 Ni₀.5 Sb₃. These phases have theskutterudite-type crystal lattice structure 40.

FIG. 13 is a graphical representation of typical Hall mobility functionsas a measure of inverse temperature for phases Ru₀.5 Pd₀.5 Sb₃ and Fe₀.5Ni₀.5 Sb₃ having skutterudite-type crystal lattice structure 40.

FIG. 14 is a graphical representation of typical Hall carrierconcentration values as a function of inverse temperature for phasesRu₀.5 Pd₀.5 Sb₃ and Fe₀.5 Ni₀.5 Sb₃ having skutterudite-type crystallattice structure 40. The electrical properties show that these phasesare heavily doped semiconductors. For example, for sample 3RPS4 of phaseRu₀.5 Pd₀.5 Sb₃ as illustrated in FIG. 12, the electrical resistivityincreases with temperature up to about 560° C. and then decreases withan activation energy of 0.25 Ev. Relatively high Hall mobilities wereachieved in phases produced in accordance with the teachings of thepresent invention in spite of the high Hall carrier concentrationmeasured for the samples. The skutterudite structure favors large Hallcarrier mobility. These two phases have a more complex structure andsignificantly lower decomposition temperature than many otherskutterudites such as CoSb₃, RhSb₃ and IrSb₃ and their thermalconductivity is substantially lower.

FIG. 15 is a graphical representation of typical Seebeck coefficientvalues as a function of temperature for skutterudite phases Ru₀.5 Pd₀.5Sb₃ and Fe₀.5 Ni₀.5 Sb₃. FIG. 16 compares thermal conductivity as afunction of temperature for skutterudite phase Ru₀.5 Pd₀.5 Sb₃ andsemiconductor material IrSb₃ with a previously available thermoelectricmaterial Bi₂ Te₃. A room temperature value of 8×10⁻³ W.cm⁻¹.K⁻¹ wasmeasured at room temperature for Ru₀.5 Pd₀.5 Sb₃, ten times lower thantypical P-type IrSb₃ which has the same crystal structure but highermelting point and bandgap.

For comparison, typical thermal conductivity values of P-type Bi₂ Te₃-based alloys were also plotted in FIG. 16. Lower thermal conductivityvalues are observed for the phase Ru₀.5 Pd₀.5 Sb₃ over the entire rangeof temperature. As temperature falls, the thermal conductivity of P-typeBi₂ Te₃ -based alloys varies as 1/T. For Ru₀.5 Pd₀.5 Sb₃, the thermalconductivity decreases as for a glassy material. Most of the crystallinematerials where low thermal conductivity is observed do not have goodelectrical conductivity. Ru₀.5 Pd₀.5 Sb₃ is a unique material where lowthermal conductivity and good electrical resistivity are combined,making this material an excellent candidate for low temperaturethermoelectric applications. This phase may, therefore, be useful in thelow temperature range (-200°, 225° C.).

Although the present invention has been described in detail, it shouldbe understood that various changes, substitutions and alterations can bemade herein without departing from the spirit and the scope of theinvention as defined in the following claims.

What is claimed is:
 1. A thermoelectric devices comprising:a firstmaterial comprising a skutterudite-type crystal lattice structure havingthirty-two atomic crystallographic sites including eight metal atomsites and twenty-four non-metal atom sites, the first material furthercomprising a first amount of a first type of dopant; and a secondmaterial electrically connected to the first material and comprising asecond amount of a second type of dopant.
 2. The thermoelectric deviceof claim 1 wherein four of said metal atom sites are occupied byRuthenium.
 3. The thermoelectric device as defined in claim 1 whereinfour of said metal atom sites are occupied by palladium.
 4. Thethermoelectric device as defined in claim 1 wherein said twenty-fournon-metal atom sites are occupied by antimony.
 5. The thermoelectricdevice as defined in claim 1 wherein the first material furthercomprises a semiconductor compound having the formula:

    Ru.sub.0.5 Pd.sub.0.5 Sb.sub.3.


6. The thermoelectric device as defined in claim 1 wherein the firstmaterial further comprises a semiconductor compound having the formula:

    Fe.sub.0.5 Ni.sub.0.5 Sb.sub.3.


7. 7. The thermoelectric device as defined in claim 1 wherein the firstmaterial further comprises a semiconductor compound having the formula:

    CoGe.sub.1.5 Se.sub.1.5.


8. The thermoelectric device as defined in claim 1 wherein the firstmaterial further comprises a semiconductor compound having the formula:

    RuSb.sub.2 Te.


9. The thermoelectric device as defined in claim 1 wherein the firstmaterial further comprises a semiconductor compound having the formula:

    PtSnSb.sub.2.


10. The thermoelectric device as defined in claim 1 wherein the firstmaterial further comprises a semiconductor compound having the formula:

    FeSb.sub.2 Se.


11. 11. The thermoelectric device as defined in claim 1 wherein thefirst material further comprises a semiconductor compound having theformula:

    FeSb.sub.2 Te.


12. The thermoelectric device as defined in claim 1 wherein the firstmaterial further comprises a semiconductor compound having the formula:

    RuSb.sub.2 Se.


13. The thermoelectric device as defined in claim 1 wherein the firstmaterial further comprises a semiconductor compound having the formula:

    RuSb.sub.2 Te.


14. 14. The thermoelectric device as defined in claim 1 wherein thesecond material comprises a material selected from the group consistingof alloys of Bi, As, Sb, Te, salts of lead with chalcogen elements,sulphur, tellurium and selenium.
 15. The thermoelectric device of claim1, wherein the first type of dopant comprises a p-type dopant.
 16. Thethermoelectric device of claim 1, wherein the first type of dopantcomprises an n-type dopant.
 17. The thermoelectric device of claim 1,wherein the second type of dopant comprises a p-type dopant.
 18. Thethermoelectric device of claim 1, wherein the second type of dopantcomprises an n-type dopant.
 19. The thermoelectric device of claim 1,wherein the second material comprises a skutterudite-type crystallattice structure having thirty-two atomic crystallographic sitesincluding eight metal atom sites and twenty-four non-metal atom sites.20. The thermoelectric device of claim 1, wherein the first materialfurther comprises a semiconductor compound selected from the groupconsisting of Ru₀.5 Pd₀.5 Sb₃, Fe₀.5 Ni₀.5 Sb₃, RuSb₂ Te, CoSn₁.5 Te₁.5,PtSnSb₂, FeSb₂ Se, FeSb₂ Te, RuSb₂ Se, RuSb₂ Te, Co_(1-x-y) Rh_(x)Ir_(y) Sb₃, IrSn₁.5 Te₁.5, Fe₀.5 Pd₀.5, Sb₃,Fe₀.5 Pt₀.5 Sb₃, Ru₀.5 Ni₀.5Sb₃, and Ru₀.5 Pt₀.5 Sb₃.
 21. The thermoelectric device of claim 1,wherein the first material further comprises a semiconductor compoundselected from the group consisting of CoSb₃, RhSb₃, and IrSb₃.
 22. Athermoelectric device comprising a first material having the formula:

    Ru0.5Pd.sub.0.5 Sb.sub.3.


23. The thermoelectric device as defined in claim 22 wherein the firstmaterial further comprises a skutterudite-type crystal latticestructure.
 24. The thermoelectric device as defined in claim 22 furthercomprising a second material selected from the group consisting ofalloys of Bi, As, Sb, Te, salts of lead with chalcogen elements,sulphur, tellurium and selenium.